1. Field of the Invention
The present invention relates to a cathode for a grid-type electron tube that can take fast switching-on and switching-off of the heating voltage.
2. Description of the Prior Art
The cathodes of grid-type electron tubes are mainly directly heated cathodes. They often take the form of a cylindrical meshwork formed by two sheets of crossed wires soldered to each other. The wires are often made of thoriated tungsten. The meshwork is mounted between two supports used to lead in the heating current. The supports are formed by solid metal parts. They are rigidly fixed to the external connections of the tube, at the base Of the tube.
Other cathodes, now less frequently used, have substantially parallel wires fixed between the two supports instead of having a meshwork with crossed wires.
When the heating of this type of cathode is suddenly turned on, the wires of the cathode, which have low thermal inertia, expand as soon as the heating voltage appears. The solid metal supports expand at a slower rate. The time constant is of the order of 1 second for the cathode wires and 1 minute for the support. The cathode wires then undergo a compressive stress.
When the heating is suddenly cut off, the reverse phenomenon occurs. The cathode wires shrink at far greater speed than do the supports. The cathode wires then undergo tensile stresses. These differences in expansion ultimately cause the permanent and irreversible deformation of the cathode wires and they may then affect the control grid. This may prompt electrical sparking or short circuits between the cathode and the grid and the breaking of the tube supply circuit.
The ON-OFF cycles ultimately damage the cathode and considerably reduce the lifetime of the tube.
Solutions have already been proposed with a view to overcoming this drawback.
The heating voltage can be applied and cut off gradually or in stages. This approach has the advantage of enabling the support and the cathode wires to compensate for the differences in expansion. However, in this approach, a non-negligible amount of time is needed to reach the working temperature of the cathode. The turning on and turning off operations are not done instantaneously. This is not acceptable in certain applications.
It has also been proposed, generally in the case of cathodes using parallel wires, to interpose a helical type of spring in one of the supports to neutralize the compressive and tensile stresses that arise in the cathode wires. Their deformation is prevented by keeping them in a taut state. This spring, which is made of metal, cannot act appropriately when it is excessively heated. Its modulus of elasticity and its elastic limit deteriorate with the temperature. So that it may remain elastic, it is placed, far from the cathode wires, at the external connections of the tube at the foot of the tube. However, the further it is from the wires the weaker is its compensation effect.
The present invention is aimed at overcoming these problems of deformation of the cathode and proposes a cathode with fast switching-on and switching-off of the heating. The invention consists in using a spring to prevent the deformations of cathode wires and in placing this spring in the vicinity of the cathode wires.